Aerobic Oxidation of 2,3,6-Trimethylphenol to Trimethyl-1,4-benzoquinone with Copper(II) Chloride as Catalyst in Ionic Liquid and Structure of the Active Species

TMQ is an important precursor in industrial vitamin E synthesis. We report a “green chemistry approach” with respect to the highly selective and environmentally friendly oxidation of 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (TMQ) with molecular oxygen as oxidant and a copper catalyst immobilized in a molten salt. n-Butanol as co-solvent has a positive effect on the activity and selectivity. The structurally characterized catalyst, a 1-n-butyl-3-methylimidazolium oxotetracuprat, is formed in situ via hydrolysis of CuCl2 in the presence of imidazolium chloride. We propose a mechanism of oxidative phenolate activation at a [Cu4(μ4-O)]6+ core as electronically coupled electron acceptor, formation of a copper-bound phenolate radical anion, spin delocalization into the aromatic ring, and attack by triplet oxygen at the para position. Attack of Cu(I) as reduction equivalent at the peroxy radical, proton-mediated elimination of a copper(II)-hydroxo species, will either substitute a copper(I) site in the reduced oxo cluster or take up an electron from the reduced mixed valent cluster [Cu4(μ4-O)]6+ to regenerate the oxidized cluster as the active electron acceptor.